Frank Lampard's side have won just four of their last 13 second-tier matches following the stalemate at the CBS Arena.
Having been 10 points clear in December, Coventry will be knocked out of pole position on Monday if second-placed Middlesbrough beat Sheffield United.
Second-bottom Oxford had Will Lankshear sent off in the 77th minute, but held on for a valuable draw that leaves them four points from safety.
Millwall consolidated their grip on fifth place with a 2-0 win at play-off rivals Wrexham.
Wrexham co-owner and Hollywood actor Ryan Reynolds was in attendance at the Racecourse Ground to see Max Cleworth deflect a Femi Azeez cross into his own net for Millwall's 59th-minute opener.
Millwall did not manage a single shot on target until five minutes from full-time when substitute Josh Coburn lashed home from 12 yards.
The Lions moved within one point of third place, while Wrexham remain sixth.
Fourth-placed Hull's hot streak fizzled out with a 3-2 defeat at home to play-off chasing Bristol City.
Alfie Devine earned seventh-placed Preston's first win in five league matches as they beat Portsmouth 1-0 at Deepdale.
Leicester's miserable week hit a new low as they lost 2-1 at Birmingham.
The Foxes, who had six points deducted after breaching the Football League's Profit and Sustainability rules, fell to a third straight defeat and had Bobby De Cordova-Reid sent off before half-time.
Leicester could have few complaints as Ibrahim Osman and Jay Stansfield netted for Birmingham, either side of Abdul Fatawu's equaliser.
The Foxes have taken just one point out of the last 15 and are only out of the relegation zone on goal difference.